Subject: How to run sshd on read-only mounted root file system?
To: None <firstname.lastname@example.org>
From: Ian Zagorskih <email@example.com>
Date: 02/24/2006 23:07:09
NetBSD MAKS 3.99.15 NetBSD 3.99.15 (GENERIC) #10: Fri Feb 24 21:23:50 NOVT
2006 ianzag@IANZAG:/home/ianzag/NetBSD/kernel/GENERIC i386
I have an embedded system with root file system mounted as read-only. The
reason is to get rid of fsck course we need this system to boot as fast as
possibly and on the other hand it can be switched off at any time without a
proper shutdown. The fstab looks like this:
/dev/wd0a / ffs rw,noatime,nodevmtime 0 1
swap /tmp mfs rw,-s=2m,noatime 0 0
swap /var mfs rw,-s=4m,noatime 0 0
I.e. there's only root filesystem mounted as read only and all dynamicall
stuff is created at boot time by hands in /var and /tmp.
In order to manage this system remotely I'm using ssh so there's running sshd.
The problem is that when user is logging in sshd calls openpty(3) to allocate
a pty and, course /dev is r/o, it fails -> I cannot login. Of course, there's
"-T" option for ssh client which instructs remote sshd not to allocate a pty,
but resulting shell is rather limited so I cannot even run vi, less etc ->
this is unlikely an "administrative shell".
At #netbsd I was advised to put dynamically generated /dev somewhere on one of
my MFS temporary file systems and mout it then over original /dev with null
fs. So here we are and at boot time the following script does this:
# PROVIDE: finishlocal
# REQUIRE: mountlocal
$_rc_subr_loaded . /etc/rc.subr
echo "Setting up dynamic /var"
mkdir -m 0755 -p /var/dev /var/run /var/log /var/db /var/chroot/sshd
mkdir -m 1777 -p /var/tmp
touch /var/log/authlog /var/log/messages /var/log/xferlog /var/log/aculog
echo "Setting up dynamic /dev"
cd /var/dev && /dev/MAKEDEV maks
ln -s /var/run/log /var/dev/log
echo "Mount dynamic /dev"
mount_null /var/dev /dev
Of course option "file-system NULL" is enabled in kernel config file and this
is the first script I run before any services like sshd are started . The
system boots just fine, device nodes are created dynamically in /var/dev and
finally mounted over /dev:
/dev/wd0a on / type ffs (noatime, local)
mfs:25 on /var type mfs (synchronous, noatime, local)
mfs:30 on /tmp type mfs (synchronous, noatime, local)
/var/dev on /dev type null (local)
Now when I'm trying to login into the system with pty allocation enabled
(default) ssh client just hangs:
$ ssh -t firstname.lastname@example.org
...here's nothing i.e. no response on any kdb events and so on. Nothing
appears in logs, console etc - just hang up. At the same time, from another
shell it looks like this:
$ ssh -T email@example.com
ps: warning: /var/run/dev.db: No such file or directory
USER PID %CPU %MEM VSZ RSS TTY STAT STARTED TIME COMMAND
root 479 7.4 4.0 316 2440 ? Ss 11:07PM 0:00.27 sshd: toor@notty
root 136 4.0 1.2 148 728 ? Ss 11:07PM 0:00.04 -sh
root 0 0.0 6.5 0 3980 ? DKs 11:03PM 0:00.01 [swapper]
root 1 0.0 1.1 48 632 ? Is 11:03PM 0:00.05 init
root 2 0.0 6.5 0 3980 ? DK 11:03PM 0:00.00 [atabus0]
root 3 0.0 6.5 0 3980 ? DK 11:03PM 0:00.00 [atabus1]
root 4 0.0 6.5 0 3980 ? DK 11:03PM 0:00.01 [pagedaemon]
root 5 0.0 6.5 0 3980 ? DK 11:03PM 0:00.02 [ioflush]
root 6 0.0 6.5 0 3980 ? DK 11:03PM 0:00.01 [aiodoned]
root 14 0.0 6.5 0 3980 ? DK 11:03PM 0:00.00 [physiod]
root 25 0.0 1.2 216 740 ? Ss 11:03PM 0:00.09 mount_mfs -s 4m -o
root 30 0.0 1.0 216 580 ? Is 11:03PM 0:00.04 mount_mfs -s 2m -o
root 378 0.0 1.2 172 736 ? Ss 11:03PM 0:00.22 /usr/sbin/syslogd -s
root 404 0.0 1.3 612 756 ? Is 11:03PM 0:00.01 /sbin/dhclient -q rtk0
root 434 0.0 2.8 284 1668 ? Ss 11:03PM 0:01.55 /usr/sbin/sshd
root 463 0.0 4.0 324 2432 ? Is 11:06PM 0:00.27 sshd: toor@ttyp0
root 475 0.0 1.1 80 636 ? R 11:07PM 0:00.00 ps aux
root 444 0.0 1.2 148 720 ? Is 11:06PM 0:00.06 -sh
I.e. at the first time I actually can access remote host, login and so on
except it hangs at some point.
The problem can be "fixed" quite easily: When I do not mount /var/dev
over /dev with NULL FS but run root read/write accessible with the same set
of devices, all works just fine as usually fine.
Well, the question is: am I right in my feeling that special files are not
accessible over NULL FS?
The second question is: when I do all the same but with UNION FS simple access
to dev like "ls -l /dev" makes a kernel panic complaining to "locking to
itself". Though I haven't found why.